The invention relates to a process according to the preamble of claim 1 and to a circuit arrangement for carrying out the process.
In department stores, goods are protected from theft by attaching to them resonance labels which have to be deactivated on payment at the checkout, or may have to be deactivated only on acceptance of the goods at the packaging counter after presentation of the checkout slip. If the label is not deactivated, this is detected by a monitoring system which is arranged in the exit area and which excites the labels to produce resonance and detects a non-deactivated label from the resonance.
Deactivation is conventionally effected by exciting the label with about 10 times its power, which causes the oscillating circuit or its capacitor to burn through. The high surge current may however also trigger an alarm at the exit of the department store, especially if the checkout is arranged sufficiently close. This danger can be eliminated only by expensive circuitry. Another difficulty is that such high transmitter powers (and such a high power is in fact involved in this case) require approval from the Federal Communications Commission or another Government authority and this is difficult to obtain (if it can be obtained at all). In order to eliminate false alarms, the monitoring means can be synchronized with the checkout in such a way that monitoring is interrupted at the instant when the checkout transmits a deactivating surge current. Particularly where there are several checkouts, this would mean that the monitoring means will in the end be switched off more than switched on, so that a thief with stolen goods may nevertheless be able to slip through.